Uncooked dry or semi-dry sausages are known to consumers under names such as salami, Mettwurst and Cervelatwurst. Generally, these are products in which the meat, after the stuffing process, has passed through fermentation with a great loss of moisture. Use is made of coarsely minced pork, beef, bacon and also curing salt and spices. After the mixture has been stuffed into suitable casings which are permeable to water vapor, the sausages are first subjected to what is termed reddening by warming. The temperatures can be highly variable (“sweating” at 18-25° C., “pasteurizing” at 60-70° C.). Subsequently, cold smoking generally proceeds, followed by ripening for several weeks in specially climatically controlled chambers (max. 23° C.). In the course of this the sausages lose between 25 and 45% of their original water content. Under the casing, owing to the action of smoke and drying, a dark-colored, sliceable surface forms. Uniform color and consistence of the surface are very important for consumer acceptability.
For unimpeded process of ripening and, as a result thereof, a uniform surface development, the sausage casing used must have high and uniform permeability to water vapor and smoke aromas. Traditionally, usually, use is made of casings made of collagen or fiber-reinforced regenerated cellulose. These casings are manufactured by industry directly in tubular form, and therefore do not have seams or other join zones and exhibit constant properties over the entire periphery.
Occasionally, use is made also made of casings having a seam joint running in the longitudinal direction. For producing sausages having a special non-cylindrical form, casings are even produced in which two fabric layers are sewed to one another at the periphery. In this manner, sausages having curved contours, e.g. pear shapes, may be made. An advantage of these casings, is the use of conventional readily joinable flat materials. For the uncooked dry or semi-dry sausage application, in this case, use is made of special fibrous flat materials, in particular woven fabrics, knitted fabrics and nonwovens, generally in coated form. Such casings are described, for example, in DE-A 21 28 613; DE 23 30 774; DE 30 29 028; DE 31 47 519 (whose United States equivalent is U.S. Pat. No. 4,525,418); DE 41 41 924; DE 42 37 138 or DE 44 17 244.
Despite all these efforts of industry, textile-based uncooked dry or semi-dry sausage casings have achieved only limited importance in the market.
However, for economic reasons, there is in addition fundamental interest in casings based on flat materials. Casings manufactured without a seam, for example those made of collagen or regenerated celluloses, are relatively expensive in manufacture. Their production proceeds with the aid of special annular dies from which a previously produced dispersion or solution of the casing material, e.g. a collagen dispersion or viscose, is extruded, and in this case is shaped to form tubular casings. The films must subsequently be passed through various chemical baths, as a result of which the material is solidified by coagulation or chemical regeneration. These processes scarcely permit rationalization of production by enlargement of the plants. High production amounts may only be achieved by parallel operation of a plurality of dies and a plurality of tube-conducting appliances. Webs of flat material, in contrast, may be produced very efficiently by producing a base material in a large web width and as required, cutting it into a multiplicity of narrower webs (what are termed blanks). A further advantage is that, starting from a standard web material, various blank widths and therefore various calibers can be achieved in a short time. This is very desirable, in particular, for the production of sausage casings, since the calibers thereof are scarcely standardized.
A further economic aspect which argues for the use of flat materials in the production of the casings relates to the stuffing operation at the sausage manufacturer's. Tubular casings cannot, in principle, be stuffed continuously. The most efficient stuffing process for tubular casings is the operation using filling and clipping machines. In this case the casings must be processed before use into what are termed shirred sticks. The length of a shirred stick is limited by the length of the stuffing horn which is provided. Stuffing horns generally have a length of 40 to 100 cm. A shirred stick—depending on its length and the wall thickness of the casing—holds about 15 to 50 m of tube material. On the filling and clipping machine, at the start of work, a shirred stick is manually pushed onto the stuffing horn. Filling, sealing and separating the sausages then proceeds automatically. If a shirred stick is used up, the process must be interrupted in order to position a new shirred stick.
In contrast, combined tube sealing and stuffing processes succeed with significantly fewer interruptions. Instead of shirred sticks, here, flat film material is used, which film material is fed to the process via reels having a theoretically unlimited length. In practice, typically 500 to 1000 m are situated on a reel. The process can run continuously until a reel is used up. The process comprises the following steps:                continuous feeding of the film web in the direction of a stuffing horn;        axial rounding of the web, preferably using a shaping shoulder, so that the web end concentrically encloses the horn and the longitudinal edges of the web touch or somewhat overlap one another;        continuous sealing of the web edges to one another by a heating element which warms the material in the joining area and pressed onto the horn. If the web edges are conducted so as to overlap, an overlapping sealing seam forms. Alternatively, the edges can be laid to abut and are firmly joined to one another by a sealing strip which is fed separately from the exterior and bridges the abutment;        transporting the tube thus formed off the horn end where it is tightly stuffed by the exiting meat mix;        dividing off portions from the stuffed tube by means of a tie-off device, fixing and sealing the tied-off links by two metal and/or plastic clips positioned next to one another, finally separating the tube between the two clips, forming individual sausages.        
The devices for this process type are known under various manufacturer-specific names, e.g. “TSCA” for equipment from Polyclip. Their structure is described in detail in various publications, e.g. in DE 14 11 489 and EP 0 105 558 (whose United States equivalent is U.S. Pat. No. 4,563,792).
Overall, therefore the tube sealing and stuffing technique gives two economic advantages over the use of seamless casings:                1. more rational production of the casing material by using a prefabricated wide web material and cutting blanks as required;        2. more efficient stuffing process, since processing interruptions are only required when a complete film reel is used up.        
In order that the seal bond can occur, one or, better, both of the surfaces which are brought together must have thermally adhesive coatings. In the case of the overlap sealing, front and rear sides of the web material must be brought together. Here, this generally concerns surfaces of different materials; in the case of textile supports, usually the bare woven fabric or nonwoven structure is usually at the rear, whereas at the front a polymer surface coating of thermoplastic character is present. In the event of a sufficiently thick and plastic surface coating, during the warming and pressing, the coating begins to melt and partial penetration of the polymer into the pores of the opposite support occurs. After cooling, the surface layer is then mechanically anchored to the opposite side. In the case butt joining using a sealing strip, customarily the sealing strip contacts by its thermoplastic surface onto the likewise thermoplastic surface of the support material. On warming, the polymers of both surfaces begin to melt and adhere to one another. Ideally, interdiffusion of both layers occurs; this occurs, in particular, when the polymers are identical or chemically related.
However, the economic advantages of the tube sealing and stuffing process are counteracted in uncooked dry or semi-dry sausage manufacture by a technical defect. The seam join running longitudinally (hereinafter also termed the sealing seam) has in principle a significantly lower permeability than the adjacent material. This is because the permeability of a medium for water vapor, as also for (smoke) gases is subject to fixed laws. A given layer material has a substance-specific permeation coefficient P according to the following equation (source: S. Pauly in: Polymer Handbook, J. Brandrup, E. H. Immergut and E. A. Grulke (editors), 4th edition (1999), volume VI, page 543, John Wiley & Sons Inc.):P=(amount of permeant)×(film thickness)/((area)×(time)×(pressure drop across the film))On rearrangement this gives:permeation=(amount of permeant)/(time)=(P×(area)×(pressure drop))/(film thickness)
The permeation is therefore inversely proportional to the thickness of the layer material.
Then in the case of sealing seams, regardless of whether constructed as an overlapping seam or butt seam with sealing strip, a substantial increase of the overall material thickness cannot be avoided in principle. In the case of an overlapping seam the material thickness is doubled (and the permeation halved). In a butt seam having a sealing strip the same applies if the thickness and composition of the strip are identical to that of the web material which is present. The effect may theoretically be reduced by a strip of lower thickness. However, in practice, this is scarcely possible, since the strip (in particular in the cross direction) must withstand the same mechanical tension as the remaining tube.
A consequence thereof is that the permeability to water vapor, as also to smoke components, under such sealing seams is only about half as great as in the adjoining casing material. In the case of an uncooked dry or semi-dry sausage filling, the sausage surface underneath the sealing seam would dry in a greatly retarded manner and an unacceptable light longitudinal stripe would form on the sausage.